In transportation-related sections and aerospace fields, active development of carbon-based materials has been conducted in recent years, for the purposes of energy saving based on weight reduction and CO2 emission control. Furthermore, SiC, which is a compound of carbon and silicon, is also acknowledged as an engineering ceramic and power semiconductor with very high characteristics and is already developed to a practical level. The advantages of the carbon-based materials include light weight, high strength, high specific strength, good heat conductivity, and, high heat resistance because of the high melting point of carbon itself.
Such materials are generally manufactured in a high temperature range over 1800° C. On this occasion, stable manufacture at high yields becomes possible with execution of temperature control during manufacture and thus it is an urgent necessity to develop a temperature measuring method in the high temperature range. A conventional temperature measuring method is a method using a radiation thermometer. However, temperature measurement is impossible in situations where an obstacle lies between the radiation thermometer and a temperature measurement target. In addition, it has another problem that temperature measurement accuracy considerably worsens in situations where infrared rays radiated from the temperature measurement target are absorbed by gas or the like.
As a solution to the foregoing problems of the radiation thermometer, there is a method making use of a thermometer using a thermocouple. Temperature measurement by the thermocouple is a method of combining wires of dissimilar materials to measure a thermoelectromotive force generated by a temperature difference in a circuit. In the temperature measurement by the thermocouple, the thermocouple can be installed in the vicinity of the temperature measurement target because the thermocouple consists of the wires, and thus it can measure temperatures with accuracy.
A tungsten-rhenium (WRe) thermocouple is known as a thermocouple for high-temperature measurement and can measure ultra-high temperatures around 3000° C. in vacuum, reductive, and inert atmospheres. An iridium-iridium rhodium (Ir—IrRh) thermocouple can measure temperatures up to around 2200° C. even in an oxidative atmosphere if the measurement is performed in short time, as well as in the vacuum, reductive, and inert atmospheres. However, under circumstances where the thermocouple is exposed to a carbon-existing atmosphere at high temperatures or under circumstances where the thermocouple touches carbon at high temperatures, the wires of the thermocouple react with carbon to change the thermoelectromotive force and, for this reason, it is difficult to use the thermocouple without protection for the wires.
It is thus common practice to provide a protection tube surrounding the thermocouple, to protect the thermocouple. For example, Patent Document 1 discloses the protection tube surrounding the thermocouple, which is the protection tube having a double-layered structure wherein portions constituting an inner peripheral wall and an outer peripheral wall of the protection tube have different compositions. In the protection tube of Patent Document 1, the inner peripheral wall of the protection tube is comprised of a material such as silicon nitride (Si3N4) with excellent heat resistance and high thermal conductivity. The outer peripheral wall of the protection tube is comprised of a ceramic material such as mixture of magnesia (MgO) particles and carbon (C) particles poorly reactive with temperature measurement objects. The inner peripheral wall and outer peripheral wall of the protection tube are comprised of respective sintered bodies, which are obtained by such integral firing as to adhere the inside sintered body and the outside sintered body to each other.